Many computer graphics systems require realistic three-dimensional models to render arbitrary, realistic but synthetic scenes. The models can be used for animation, solid modeling, visualization, robotics, games, scientific studies, CAD/CAM, and multimedia. In computer graphics systems, three-dimensional rendering software and graphics luminance models typically display three-dimensional models.
Due to many limitations and incomplete information about the physical objects to be modeled, it is very difficult to derive complete mathematical models and supply all necessary camera parameters. Generating high complexity models is therefore tedious and extremely time-consuming.
Systems which use laser scanning and range sensing hardware to generate 3D information from an object are very expensive and therefore inaccessible to most people. In addition, the complex hardware is difficult to repair or replace. Laser scanner methods and systems are also intrusive. The color map from laser systems is also restricted by the wave-length of the laser light and therefore lacks realism.
Other methods acquire images of the object from a discrete number of vantage points. The images are stitched together to form a panoramic screen of the object. However, because no model is ever generated, one cannot place the object in an arbitrary scene, and the object can be viewed only from the vantage points at which the images where acquired.
Many systems use geometrical camera calibration to determine a set of camera parameters. The camera parameters describe the mapping between the three-dimensional coordinates of the object and the two-dimensional image coordinates. The numerous calibration techniques known are typically divided into two groups, namely, one-time, high-accuracy parameter calculation methods, and task-oriented, coarse, inexact calibration methods.
The high accuracy methods rely on an accurate determination of image features, and typically use specific assumptions about the environment or camera parameters to achieve better accuracy. These computationally expensive calibration techniques are usually performed only once, as a preprocessing step of a model acquisition process.
One accurate calibration technique is described by Tsai in “A Versatile Camera Calibration Technique for High-Accuracy 3D Machine Vision Metrology Using Off-the-Shelf TV Cameras and Lenses,” IEEE Journal of Robotics and Automation. Vol. RA-3, No. 4, 1987. Tsai's camera model is based on a pin-hole model of 3D-2D perspective projection with first order radial lens distortion. His technique accounts for camera pose in a world coordinate system with 3D rotation and translation, scaling, focal length, and aspect ratio.
Coarse calibration techniques are typically used in complex, interactive environments. As an advantage, coarse calibration can provide continuous calibration updates, simplicity, quick computation, and adaptation to changing camera and world conditions.
In applications where the three-dimensional position and orientation of the camera images relative to the object are arbitrary, the object is sometimes imaged against a calibration pattern in the background. Typically, the pattern includes a number of horizontal and vertical grid lines having known spacing. Other patterns can include dots or other markers, again arranged in a known rectilinear relationship. There are a number of problems with such systems.
To acquire accurate models of real world objects, turntables can be used. Here, the object is placed on a rotating turntable, and the object is imaged from various directions, see U.S. patent application Ser. No. 09/842,972 “Image-Based 3D Digitizer,” filed by Pfister et al., on Apr. 26, 2001. Single-camera turntable-based scanners have a long history, see Szeliski, “Shape from rotation,” Proc. Conf. Computer Vision and Pattern Recognition, 1990. However, turntable-based systems that use multiple scanners have received little attention, and the task of calibrating such systems is difficult.
Therefore, it is desired to provide a calibration technique for calibrating multiple cameras for modeling systems where objects are placed on turntables for 3D model acquisition.